Steam turbines, for example, intermediate pressure steam turbines, can be susceptible to solid particle erosion (SPE). Solid particle erosion occurs when solid particles within the steam flowing through the turbine impact the rotating and stationary turbine component parts. The solid particles within the steam can cause erosion of the static blades (or nozzles), rotating blades (or buckets) and tip sealing devices that are in sealing relationship with the shrouds at the tips of the rotating blades. Although solid particle erosion can occur at any point along the steam flow path through a steam turbine, it can be prevalent in the early turbine stages of an intermediate pressure (IP) steam turbine.
Erosion of the trailing edge region of the static blades of a turbine stage can be a particular concern and is known to be caused by rebound of the solid particles off the rotating blades of that particular turbine stage, in a direction opposite to the steam flow direction through the turbine. Known responses for reducing this particular type of solid particle erosion are described in U.S. Pat. No. 4,776,765. One response is to provide a coating or sheet of protective material on the trailing edge of the static blades of a turbine stage to minimise the susceptibility of those blades to solid particle erosion due to rebound off the adjacent rotating blades of the turbine stage. Another response, which can be used alone or in combination with the aforesaid protective material, is to increase the spacing between the static blades and the rotating blades of a turbine stage to thereby reduce the momentum of any rebounding solid particles.
The responses proposed in U.S. Pat. No. 4,776,765 seek to address erosion to the trailing edge of the rotating blades of a turbine stage whereas, as mentioned above, solid particle erosion can occur at any point along the steam flow path through the turbine. While it may be desirable to reduce solid particle erosion at any point in the steam flow path through a steam turbine by eliminating the solid particles from the steam flow before the steam reaches the turbine, this is impractical. Other responses have, therefore, been proposed.
One response, described in U.S. Pat. No. 4,726,813, utilizes electromagnets, arranged on the piping connecting the boiler to the turbine, to create a magnetic field and thereby deflect solid metallic particles within the steam flow to a desired location where they are collected. The steam then proceeds to the steam turbine for expansion through the turbine stages.
Another response, described in U.S. Pat. No. 7,296,964, is to divert a proportion of the solid-particle-containing steam flowing through the steam turbine away from the main steam flow path, to the feed water heater of the turbine. The diverted steam thereby bypasses downstream rotating components. Holes and passages can generally be provided in the component parts of the steam turbine to permit the diversion of a proportion of the solid-particle-containing steam and in one embodiment, holes and passages can be provided in the radially outer static ring of the first turbine stage. These holes and passages communicate with a passage in the radially outer static ring of the second, downstream, turbine stage to divert a proportion of the steam away from the rotating blades and blade tip sealing devices of the first turbine stage into a steam extraction passage to the feed water heater.
The responses described in both U.S. Pat. No. 4,726,813 and U.S. Pat. No. 7,296,964 are complex and may not always provide a sufficient reduction in the level of solid particles contained within the steam flow. The complexity of the response proposed in U.S. Pat. No. 7,296,964 arises partly from the fact that holes and passages need to be formed through the radially outer static rings and tip sealing devices of multiple stages of the steam turbine. Furthermore, because the holes and passages are provided in the radially outer static ring at only predetermined circumferential positions, the ability to divert solid particle containing steam can be limited, thus limiting the effectiveness of the proposed solution.
Exemplary embodiments disclosed herein are directed to improved extraction of solid particles from axial flow steam turbines to render them less susceptible to damage arising from solid particle erosion (SPE).